Account for IfUnpackCo in freeNamesIfDecl
Account for IfUnpackCo in freeNamesIfDecl

We were simply failing to recognise all the free variables of an
IfaceDecl, notably the ones in the coercion of an IfUnpackCo.
Result: the dependency analysis got messed up, so that fingerprint
calculation went wrong.  Trac #13695 showed it up.

A test case is tricky but the fix is a solid one.

sdiehl/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc